Pteridine coenzymes are those pteridine derivatives which have been reported to serve as growth factors for Crithidia fasciculata, coenzymes in hydroxylation reactions, and cofactors in still other unknown reactions. The information on the structure of these pteridine coenzymes indicates a pterin derivative (2-amino-4-hydroxypteridine) with a three carbon substituent on position 6. The pteridines associated with the pteridine coenzymes are biopterin and L threoneopterin. These compounds which have been reported to occur in mammals, amphibia, insects, protozoa and bacteria originate from a guanine compound. Accordingly, the synthesis and regulation of pteridine coenzymes will be studied in growing cell cultures derived from normal and neoplastic tissues. The cell cultures which will be used are Chinese hamster ovary and murine neuroblastoma. To study biopterin biosynthesis, experiments will be carried out using radioactive precursors in growing cell cultures, enzyme preparations from mammlian cells, and auxotrophic mutants of Chinese hamster and neuroblastoma cells. Standard cell culture techniques will be employed for the growth of cells; published procedure will be followed for the isolation of auxotrophic mutants; conventional methods will be used for enzyme studies; radioactive tracer technique, spectrophotometry and chromatography and a new procedure using high pressure liquid chromatography will be developed for pteridine analyses.